Evaluating Potential for Weighing Rain Gauges on the Kansas Mesonet

Measuring precipitation is one of the most challenging observations taken by meteorological instrumentation. The Kansas Mesonet, a network of more than 85 weather stations has been testing Apogee Cloudburst SG-420 weighing rain gauges (WG) to increase the accuracy and reliability of precipitation measurements. These are non-heated gauges that consist of a 6.3” orifice that measures moisture with a load cell that weighs the mass within it. The goal of this project is to determine if the lower-cost WG is an adequate consideration to replace the 100+ tipping bucket rain gauges currently deployed. Tipping bucket gauges are issue-prone, often becoming clogged and underestimating measured heavy and frozen precipitation. We co-located a WG at 13 locations, of which all but two have dual tipping bucket rain gauges installed. These dual gauges consist of Texas Electronics TE-525s (TB) and Hydrologic Services (now Kisters) siphon TB3/3H-LPs (SG). These makeup dual gauges at each station with one gauge categorized as “Primary” (the gauge that reports to the main Mesonet server/webpage) and “Secondary” (the backup available to backfill if for some reason the Primary fails and for additional quality assurance checks). While primary gauges vary in rain gauge type, secondary gauges are all TB. Five stations in this study have the SG, all acting as Primary. Lastly, all rain gauges for each station are within an alter shield with tops approximately 30” above the ground. The WG’s orifice resides approximately 31-32” off the ground, about 1-2” higher due to mounting limitations within the same type of alter shield.

This project analyzed 523 independent rainfall events from March to September 2024. Initial results show several trends between the accuracy of the WG, TB, and SG rain gauges. Based on the initial performance analysis, the weighing gauges have shown a tendency to report ~ 3-5% higher average precipitation compared to the (TB) and (SG). One of the standout findings from the first six months of testing is the WGs performance during heavier rainstorms. When precipitation exceeds in a 5-minute period (or 1.8”/hour rate), the WG appears to capture higher rainfall totals than the TB/SG. This suggests that the load cells in these gauges are less prone to the underreporting issues commonly observed in tipping bucket gauges. Some potential results of underreporting may be over-spill or mechanical limitations of the bucket-tipping mechanism. As rain rates increase, the lag between tips can lead to splash out, resulting in underreported totals. Since WG doesn’t have any mechanical parts and a large catch basin, this appears to better measure heavy precipitation events. This is especially critical for water resource management and flooding analysis which must have accurate precipitation data.

The WG has shown comparable performance to the TB/SG in light rainfall conditions, consisting of about 70% of all events recorded. With precipitation event totals less than 0.15” there was a 97% correlation , with only slight differences noted. The WG measures around 1% more precipitation on average during these events. Potential light precipitation event error sources in the TB/SG consist of water remaining in the tipper or syphoon, small enough that it fails to tip/measure. However, this amount could result in differences of 0.01” per event . This is very similar to the average observed difference between WG and TB/SG during light rain events, 0.02”. Therefore, WG’s enhanced sensitivity may offer a marginal advantage in capturing the small variations.

Collected data has been reliable across the WGs with performance and quality being maintained through the period. However, the adoption of new technology has come with several lessons. Physically supporting the gauge has been important in maintaining quality data during wind events. Early gauges weren’t installed in concrete. This resulted in erroneous data during periods of increased wind. Additionally, one gauge had daily recorded precipitation of 0.02” with no moisture in the area. After a routine maintenance visit, this issue disappeared. Therefore, it is thought that excessive vegetation could have been the cause of the erroneous noise.

The warm season performance of the WG has proven to be reliable and similar compared to the current Kansas Mesonet TB/SG rain gauges. Observed precipitation during both heavy and light rain events exceeded that of the older style gauges in this study. This appears to be the result of measurements at times when both tipping-style gauges are known to struggle. Therefore, consideration of a WG to the Kansas Mesonet suite of instrumentation is warranted. However, additional assessment is needed to evaluate performance during winter precipitation events. Although this is the driest time of the year for Kansas, quality data of frozen precipitation is limited across the region.